exchange 2000

i have an exchange 2000 server at the head office and at the branch offices. The parent domain is at the head office and the child domains at the branches. There are times the branch office users cannot commmunicate with the head office by e-mail, but can communicate within the branch. The branch office users wouldn’t be able to see the global address list but the list within the branch. How can i resolve this so that they can communicate across branches at with the head office